As illustrated in FIG. 9, an automobile steering apparatus is constructed so that the movement of a steering wheel 1 is transmitted to a steering gear 2. More specifically, the movement of the steering wheel 1 is transmitted to an input shaft 6 of the steering gear 2 by way of a steering shaft 3, a universal joint 4a, an intermediate shaft 5, and a universal joint 4b; and the steering gear 2 applies a desired steering angle to the steered wheels by pushing or pulling tie rods 7. In this steering apparatus, an electric-powered power steering apparatus is assembled such that an electric motor 8 applies an auxiliary force to the steering shaft 3 according to a force that is applied to the steering wheel 1 by the driver.
In order to be able to adjust the forward/backward position of the steering wheel 1 according to the size and driving posture of the driver, this steering apparatus has a mechanism that causes a steering shaft 3 and a steering column 9 that rotatably supports the steering shaft 3 to extend or contract. More specifically, the steering shaft 3 is constructed by a so-called telescopic steering shaft that combines an outer shaft 10 and an inner shaft 11 by a spline joint so that extension and contraction is possible, and so that torque can be transmitted. Moreover, the steering column 9 is constructed by an outer column 12 and an inner column 13 that are combined so as to be able to freely extend or contract.
The intermediate shaft 5 also has extendable and contractible construction in order to prevent the steering wheel 1 from being pushed to the driver's side during a collision accident. FIG. 10 illustrates an example of construction of a conventional intermediate shaft. The intermediate shaft has an inner shaft 15 that is provided with a male spline section 14 around the outer-circumferential surface of the front end section thereof, and a cylindrical outer shaft 17 on which a female spline section 16 is formed around the inner-circumferential surface thereof so that the male spline section 14 can be freely inserted therein. By fitting the male spline section 14 into the female spline section 16 with a spline fit, the inner shaft 15 and outer shaft 17 are combined so as to be able to freely extend or contract. The base-end sections of one of the yokes 18a, 18b of the universal joints 4a, 4b are welded and fastened to the base-end sections of the inner shaft 15 and outer shaft 17.
As illustrated in FIG. 10, the intermediate shaft 5 is such that normally the male spline section 14 on the front-end side of the inner shaft 15 is located in the portion near the rear end of the female spline section 16 of the outer shaft 17. When the front section of the vehicle is pushed and crushed and the steering gear 2 is pushed toward the rear due to a collision accident (primary collision), the entire length of the intermediate shaft 5 contracts by the inner shaft 15 (male spline section 14) staying in the original position as is and the outer shaft 17 moving toward the rear, so that the male spline 14 enters the innermost section of the female spline section 16. With this kind of construction, even though the steering gear 2 is displaced toward the rear, the steering wheel 1 is prevented from moving toward the rear and being pushed toward the driver's side, so it is possible to protect the driver during a primary collision. During a secondary collision, the entire length of the steering shaft 3 contracts, which makes it possible to protect the driver from colliding with the steering wheel 1.
For an extendable and contractible shaft for a steering apparatus such as an intermediate shaft 5 and steering shaft 3, (1) a function for allowing torque to be transmitted during steering without any looseness even though there is backlash in the engaging section between the outer shaft and the inner shaft, and (2) a function that allows the shaft to extend or contract with a small force when extending or contracting the shaft are desired. Therefore, a synthetic resin coating layer that slides easily (has a low coefficient of friction) is provided on at least one of the outer-circumferential surfaces of the male spline section of the inner shaft and inner-circumferential surface of the female spline section of the outer shaft as disclosed in JP 2011-173464 (A), JP 2005-042761 (A), JP 2012-040949 (A) and the like.
When providing a coating layer on the outer-circumferential surface of the male spline section of the inner shaft, a resin layer is formed by forming a synthetic resin coating on the outer-circumferential surface of the male spline section, and then the excess portion is removed to make the film thickness of the resin layer uniform, and a finishing process is performed in order to improve the surface roughness. When performing the finishing process, it is important that the phase of the unevenness in the circumferential direction of the male spline section and the phase in the circumferential direction of the cutting tool such as a shaping cutter, broach or the like precisely match. When the shift in the phases in the circumferential direction of the male spline and cutting tool is large, there is a possibility that the thickness in the circumferential direction of the coating layer becomes uneven.
As illustrated in FIG. 11, in order to align the phases in the circumferential direction of the male spline section and cutting tool, the overall length of the male spline section 14a can be made to be long, and the male spline section 14a can be provided up to the portion where there is no engagement with the female spline section 16 of the outer shaft 17. In other words, when performing the finishing process after a resin layer has been formed on the tip-end section and intermediate section (sections other than the portion near the base-end section) of the outer-circumferential surface of the male spline section 14a, the phases in the circumferential direction of the male spline section 14a and the cutting tool are aligned by the portion near the base-end section of the male spline section 14a that is not covered by the resin layer. As a result, it is possible to form a synthetic resin coating layer 19 that has a uniform thickness and good surface roughness on the outer-circumferential surface of the portion of the male spline section 14a except for the portion near the base end. However, in the construction illustrated in FIG. 11, the male spline section 14a is provided up to a portion where there is no engagement with the female spline section 16, so the weight of the inner shaft 15a increases.
JP 2011-173464 (A) discloses technology in which a tapered section is provided on the tip-end section of the male spline teeth of the male spline section so that the thickness and depth of the male spline teeth become smaller going toward the tip-end edge, and the spline shaft having this kind of male spline section is pushed inside the broach in a state in which relative rotation in the circumferential direction is possible, after which a finishing process is performed on the resin layer. In this case, the male spline section is guided by engagement between the tapered section and the uneven section that is formed on the inner-circumferential surface of the broach, and it is possible to align the phases in the circumferential direction of the uneven section in the circumferential direction of the male spline section and the uneven section that is formed on the inner-circumferential surface of the broach. However, alignment of the phases in the circumferential direction of the male spline section and broach is performed by using the tapered sections in a state in which a resin layer has been formed on the outer-circumferential surface of the tapered section of the male spline section, so there is a possibility that this phase alignment will not be performed precisely. Moreover, the tapered section of the male spline section does not engage with the female spline section after completion and is essentially an unnecessary portion, so the weight of the inner shaft further increases by the amount where the tapered section is formed.